1. Field of the Invention
This invention relates to video transmission systems. More particularly, this invention relates to methods and apparatus for multiplying a video signal by transmitting the signal through an AC power line.
2. Description of the Related Art
Various arrangements for multiplying the number of potential receivers of a video signal have been devised. The most common arrangement of multiplying the number of receivers of a particular video source, such as a Video Cassette Recorder (VCR), includes independently wiring the output of the video signal source to each of the desired receivers. In a home environment, for example, the video output of a VCR in one room of the home is independently wired to a television receiver in another room of the home through a coaxial cable extending between the television receiver and the VCR. If the user wishes additional television receivers to receive the VCR video output, the video output of the VCR is coupled via a coaxial cable to a splitter and the respective outputs of the splitter are independently wired to the respective television receivers through a plurality of additional coaxial cables. In such an "independently wired" arrangement, the user is forced to wire the home with a network of coaxial cables to deliver the VCR video output to the various locations in the home where the respective television receivers are situated.
With respect to audio signals from VCRs and other audio signal sources, devices are available which transmit the audio signals through the existing home power line to eliminate the requirement of independently wired connections between the audio signal receivers, such as speakers and intercoms, and the audio signal source. With these devices, the user may plug the audio signal source directly into an existing power outlet through an adapter and may then connect any number of audio signal receivers to the audio signal source by plugging each audio signal receiver into any existing home power outlet through another adapter. While these prior devices connected the audio signal receivers to the audio signal source through the existing home power line, prior attempts to transmit video signals through the existing home power line have been wholly unsuccessful.
While the desirability of transmitting video signals across existing home power lines is a widely sought objective, realization of the goal has been elusive due to numerous problems, including the problem of matching the transmitter and receiver impedances to the impedance on the power line. The impedance on the power line is constantly changing as loads are added to and removed from the power line. The 75 ohm impedance at the VCR output and at the television receiver input will not ordinarily match the impedance on a home power line, since the impedance on the home power line may vary, for example, from 10 ohms to 1000 ohms. Due in part to inadequate impedance matching, signal attenuation caused by the mismatched impedances will substantially diminish the video signal quality and prohibit transmission of the video signal across the home power lines.
Even with respect to prior audio signal transmitters, impedance matching is a significant problem. Since the power line impedance so significantly fluctuates, attenuation of the audio signal on the power line is dramatic. To compensate, the prior audio signal transmitters amplified the audio signal to extremely high levels, for example 10 Watts at 100 KHz in a home system, in order to compensate for the expected attenuation loss on the unmatched power line. With respect to a video signal, however, simply boosting the power to a level overcoming the greatest expected attenuation level on the power line is problematic. First, boosting the power level of the video signal results in substantial power consumption, most of which is simply lost on the power line due to video signal attenuation. More importantly, in the event that the power line impedance and the receiver impedance coincidently match, the highly boosted video signal will suffer little attenuation loss on the power line, causing the power line to broadcast the low-attenuation, high-power video signal to potentially distant regions outside of the power line intended to carry the video signal.